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Tsumiki is a short girl with long purple-blue hair that reaches her ankles. Her hair is tied into a very low ponytail with a fat white scrunchie. She has sleepy, purple eyes.

She forms cat ears and a cat tail from time to time, often when she sees Io. At all other times, though, she has a large, swirly ahoge on the top of her head. She usually wears the Nekoge Prefectural High School uniform, and is shown wearing the version with a sweater during the winter (as opposed to wearing the winter version with a jacket).

Tsumiki has a bit of a hot-cold personality, and is said to be a tsundere by Mayoi, although she is closer to a kuudere. She is mean and violent towards Mayoi, and hurts Mayoi whenever she teases her and Io as a couple. She often tries to hide her feelings and comes off as distant. She cares for her friends, however, as it is shown where she forgets her chocolates, but tells Hime to give hers anyways, since Hime put in the effort.

Tsumiki is often short-tempered towards Mayoi, who constantly teases her about her feelings for Io. When Io gets confused about her advances and makes her feel embarrassed, she bites him on the head.

She is much more of a tsundere in the manga, where she often strikes Mayoi over mild teasings and seems to bite Io's head a lot more than she does in the anime.

Tsumiki is also quite aggressive, as she bites Io on the head, strikes Mayoi and flings the latter into walls, started hitting Mayoi because of her bust size, and does things like squishing Hime's face.

Io Otonashi - Io is the target of Tsumiki's intense crush, though he almost always fails to see the signs, even when Mayoi teases both of them about it, much to her dismay and frustration. Despite this, Io's attitude and disposition towards her is a lot friendlier than towards other characters. Io thinks Tsumiki is cute, and often gives her a pat on the head, which she enjoys greatly.

Mayoi Katase - Mayoi often teases Tsumiki about her crush on Io, which, more often than not, earns her a devastating blow from the former. However, they both care about each other and are always seen hanging out.

Hime Haruno - Hime is Tsumiki's close friend, whom she always seem to be around. Hime is one of the few people who have never (purposefully) teased Tsumiki about her crush on Io, thus making her much less of a target than the runny-mouthed Mayoi, though in the manga Tsumiki has threatened to hit her and "squished" Hime's face when she found out that Hime was hiding Io's part-time job.

Sakaki Inui - Sakaki often hangs out with Tsumiki, though the two did not really share any interactions with each other, as Sakaki is more commonly seen around Io or Mayoi. Sakaki knows about Tsumiki's crush, and sometimes attempts to help by persuading the emotionally-dense Io to go out with her.

In both anime and manga publications of the series, Tsumiki's ahoge disappears when her cat ears come up. During the entire anime she is pictured with either the ahoge or the cat ears, however, in the manga it is not always present.

Tsumiki displays an unusual level of strength for her size, often demonstrated at Mayoi's expense. She also seems to have some mysterious powers. However, this is never acknowledged in the anime. Her odd magical powers are never acknowledged either. This is most likely due to them being considered gags.

She is shown to be a good cook and a smart student, placing first in class ranks in terms of academics.

She makes cat noises when she flusters, fitting into the series' cat theme.

She is left-handed.

She resembles Konata Izumi from Lucky Star because they both have blue hair with an antenna and eyes that are mischievous and sleepy.